1. Field of the Invention
The present invention relates to color pixel structures of color display systems based on electrochromism. More particularly, the color pixel structures includes one-cell two-color type unit cells, each having an upper panel, a lower panel and electrochromic materials of different colors applied to both the upper and lower panels, thereby achieving improved color characteristics and a simple structure.
2. Description of the Related Art
Electrochromism is when the color of a material reversibly changes depending on the direction of an electric field when a voltage is applied to the material. Electrochromic materials refer to materials that exhibit electrochromism and can undergo reversible changes in optical properties by electrochemical redox reactions. That is, some electrochromic materials do not have any color when no external electric field is applied and are then colored in the presence of an electric field. However, some electrochromic materials have particular colors when no external electric field is applied and then their colors disappear in the presence of an electric field. Such electrochromic materials include, for example, tungsten oxide and molybdenum oxide as inorganic compounds, and pyridine compounds and aminoquinone compounds as organic compounds, but are not limited to these compounds.
Conventional electrochromic devices utilizing the principle of electrochromism offer the advantages of high reflectance without the need for external light sources, flexibility, portability and lightweight. Based on these advantages, conventional electrochromic devices are expected to have many applications in a variety of flat panel displays. In particular, conventional electrochromic devices have received considerable attention in recent years for their applicability to electronic paper, which is currently being investigated as an electronic medium that is likely to replace traditional paper.
FIG. 1 is a schematic cross-sectional diagram illustrating a unit cell of a conventional electrochromic display device. As shown in FIG. 1, an upper transparent electrode 10 is coated with a transparent semiconductor material 20 and an electrochromic material 30, and a lower electrode 60 is coated with a counter material 50 responding to electrochemical reactions and a reflective material 40 reflecting light.
FIGS. 2a through 2e are conceptual diagrams illustrating a principle of color representation of a conventional color display device based on electrochromism. Like a conventional color display, an electrochromic display includes a structure in which three respective unit cells emitting red, green and blue colors are arranged parallel to one another.
To represent a white color in the conventional electrochromic display device, an electric field is removed from all color display units (see FIG. 2a). Since electrochromic materials of all color display units are transparent, white light entering all display units is reflected from reflective panels, so that an observer can perceive a white color.
An electric field is applied to a red display unit and no electric field is applied to the other color units (i.e. green and blue color units) to represent a red color. As a result, the red display unit reflects incoming red light to emit a red color, and the other color units produce a white color, thus allowing the observer to perceive a red color (see FIG. 2b). The same principle as above is applied when it is intended to represent green and blue colors (see FIGS. 2c and 2d). That is, an electric field is applied to a display unit corresponding to an intended color to display the color. As for a black color, an electric field is applied to all color units to allow the respective display units to produce red, green and blue colors. Since the amount of light reflected from the display units is smaller than that in the case of white representation, the observer perceives a black color (see FIG. 2e).
When the conventional unit cells arranged in parallel are used to represent colors other than white, however, light of undesired colors enters the eyes of the observer, resulting in deterioration of color purity. For example, the red emitting unit cell represents a red color, but the other adjacent two unit cells (green and blue emitting unit cells) produce a white color. As a result, the observer perceives a color close to pink but not a red color. In conclusion, a reduction in color purity in representing images, such as images printed on paper, in the parallel type structure is inevitable.
Such a problem can be solved by the provision of laminate type cell structures, for example, structures in which unit cells emitting cyan, magenta and yellow are vertically laminated (see FIG. 3). Since only colors produced from the laminates can be viewed by an observer, substantially pure colors can be represented. However, the laminates consist of the three unit cells, making the lamination procedure complex and the operation of the cells complicated. Further, a total of six electrodes (each layer including upper and lower electrodes) are laminated, disadvantageously causing a loss of light.